In recent years, the frequency bands allocated to radio communication have been becoming saturated. To solve this saturation problem, a dynamic allocation concept called a “radio-opportunistic” system or “cognitive communication” has been studied. This principle relies on the ability to analyze a frequency spectrum to avoid busy occupied frequency bands, identify and determine available unoccupied frequency bands, and switch a communication technique. In order to implement this dynamic frequency allocation principle, however, extremely wideband oscillators and tunable filters are needed.
Reception performance (sensitivity and selectivity) of portable terminals typically depends on frequency selectivity attenuators (band-pass filters) having frequency selectivity and mixers. In particular, band-pass filters having high Q factors are demanded in order to use frequency bands effectively and perform radio telecommunication with low energy. Requirements for tunable filters are adjustability of center frequencies of the filters and control of expansion or narrowing of pass-bands of the filters. Existing resonators such as surface acoustic wave (SAW) filters, which are filters utilizing surface acoustic waves propagating along the surface of a piezoelectric material, and bulk acoustic wave (BAW) filters, which are filters utilizing resonant oscillation of a piezoelectric film called a BAW, cannot achieve the above-described requirements at the present time. Thus, tunable band-pass filters that are compact enough to be included in portable terminals have not been obtained yet.
A tunnel magnetoresistive (TMR) device including a spacer layer of a nonmagnetic material interposed between a fixed magnetic layer and a free magnetic layer is a known example of a magnetoresistance effect device. In a TMR device, spin-polarized electrons flow when a current flows, and the magnetic orientation (i.e., the orientation of an electron spin) of a free magnetic layer changes in accordance with the number of spin-polarized electrons accumulated in the free magnetic layer. When an attempt is made to change the magnetic orientation of a free magnetic layer that has been disposed in a certain magnetic field, torque will act so as to restore the electron spin to a stable orientation that is restricted by the magnetic field, and when the electron spin is perturbed with a specific force, oscillation called spin precession will occur.
Lately, there has been found a phenomenon (spin torque ferromagnetic resonance) in which when a high-frequency AC current flows in a magnetoresistance effect device such as a TMR device, strong resonance will occur if the frequency of the AC current flowing through the free magnetic layer matches the oscillation frequency of the spin precession that is attempting to restore the magnetic orientation (see Non Patent Literature 1). It is also known that when an RF current (i.e., an RF current with a frequency that matches the oscillation frequency (resonant frequency) of the spin precession) is injected into a magnetoresistance effect device in a state where a static magnetic field is applied to the magnetoresistance effect device from the outside and the orientation of the static magnetic field is inclined by a predetermined angle in the fixed magnetic layer relative to the magnetic orientation of the fixed magnetic layer, the magnetoresistance effect device will function so that a DC voltage that is proportional to the square of the amplitude of the injected RF current is generated across the two ends of the magnetoresistive device, or in other words, achieve a square-law detection function (or a spin torque diode effect). It is also known that an output of square-law detection from the magnetoresistance effect device is higher than an output of square-law detection from a semiconductor pn-junction diode under certain conditions (see Non Patent Literature 2).
The applicant of the present invention has focused on the square-law detection function of a magnetoresistance effect device, and has already proposed an application thereof to a mixer capable of operating at a low local power (see Patent Literature 1). A mixer of a magnetoresistance effect device includes a magnetic field applying unit that applies a magnetic field to a free magnetic layer, and when receiving a first high-frequency signal S1 and a second high-frequency signal S2 as a local signal, generates a multiplication signal S4 using magnetoresistance effect.
However, the multiplication signal S4 is significantly attenuated in a 50 Ω matching circuit. Thus, in order to make the impedance to the multiplication signal S4 higher than the impedances to the first high-frequency signal S1 and the second high-frequency signal S2, disposing an impedance circuit (a filter or a capacitor) between an input transfer line for transmitting the first high-frequency signal S1 and the second high-frequency signal S2 and the magnetoresistance effect device was proposed (see Literature 2).
Although the above-described phenomenon regarding the mixer of a magnetoresistance effect device is known, a high-frequency device capable of applying this phenomenon to an industrial use has not been discovered yet, and has been anticipated. The applicant of the present invention was able to find out that a multiplication signal output varies depending on resonance characteristics and a selective function in terms of frequency is present in the square-law detection function of a magnetoresistance effect device. However, a high Q factor could not be obtained, and a frequency selection range is considerably large. For these reasons, no industrial application has been found to date.